Nitridochromate(IV): LiSr2[CrN3]

The quaternary nitridochromate(IV) LiSr2[CrN3] crystallizes in a new structure type with the non-centrosymmetric space group P21 (no. 4) with a = 5.5685(7) Å, b = 5.3828(8) Å, c = 7.5381(1) Å, and β = 92.291(8)°. Predominant structural features of the compound are slightly nonplanar trigonal units [CrN3]5–, which are connected by three-fold coordinated lithium to form slabs in the (001) plane. Shorter Cr–N bond lengths in comparison with reported nitridochromates(III), as well as diamagnetic behavior and vibrational spectroscopy data indicate Cr(IV), which is in a good agreement with the charge balance. According to electronic structure calculations, the compound is a semiconductor with a band gap of 1.19 eV.


■ INTRODUCTION
Ternary and multinary nitridochromates of alkali and alkalineearth elements exhibit oxidation states between III and VI for chromium, whereas in binary chromium nitrides, only a maximum of Cr(III) at ambient pressures is observed. 1,2 Depending on the nitrogen partial pressure in the system, the highest oxidation states (V, VI) can be reached in a combination with lithium and alkaline-earth metals. These compounds typically consist of isolated tetrahedral anions [CrN 4 ] x− or edge-sharing tetrahedra with chromium−chromium bonding. Up to now, the following Cr(V) and Cr(VI) nitridometalates are known: Li 4 6 and Sr 3 [Cr VI N 4 ]. 7 Additionally, only a few compounds with Cr(III) in (distorted) trigonal planar coordination and an alkaline-earth metal as a cation are reported: Ca 3 [Cr III N 3 ], 8 N 3 ]. 9 Recent studies suggest that some of these compounds may be described by balances with excess electrons Sr 3 [Cr IV N 3 ]·e − and Ba 3 [Cr IV N 3 ]·e − , 10 which are able to attach hydrogen, forming multianionic nitridochromate-hydrides Sr 3 [Cr IV N 3 ]H and Ba 3 [Cr IV N 3 ]H, 11 respectively. Moreover, the nitridochromate−hydride Ca 6 [Cr 2 III,IV N 6 ]H, 12 which contains an ethane-like anion, provides another example for mixedvalent/intermediate valent Cr(III)/Cr (IV) species and a direct Cr−Cr interaction. However, to our knowledge, no "pure" nitridochromates (IV) are described until now. Here, we report on the synthesis and characterization of a new multi-cationic nitridochromate LiSr 2 [CrN 3 ], which contains Cr (IV), and trigonal planar units [LiN 3 ], forming slabs in the (001) plane.

■ EXPERIMENTAL SECTION
Single crystals of LiSr 2 [CrN 3 ], showing dark gray metallic lustre, were synthesized applying a modified high-temperature centrifugation-aided filtration technique (HTCAF). 13,14 A mixture of Sr 2 N, Cr, Li 3 N, and Li in molar ratio 1:1.8:2.8:17 was sealed in a tantalum ampule with integrated sieve and subsequently heated at 1023 K for 2 h. After cooling down to 573 K, the mixture was centrifuged to allow for proper separation of crystals from the flux.
To check a possible phase width and potential other phases in the system, several more experiments were performed using the same HTCAF technique, but employing different starting materials, molar ratios, or thermal regimes ( Nearly single-phase samples of dark gray microcrystalline LiSr 2 [CrN 3 ] were synthesized from pelletized mixtures of Sr 2 N, CrN, and Li 3 N with molar ratio 0.7:1:2.6 in sealed tantalum ampules at 1023 K for 36 h. Several subsequent cycles involving grinding, adding excess of Sr 2 N and Li 3 N, and re-heating at the same temperature for longer times led to nearly single-phase samples of LiSr 2 [CrN 3 ] with a small amount of unspecified impurities. High temperatures and long annealing time may lead to evaporation of light elements and decomposition of alkaline-earth nitrides, as well as reactions with the crucible material; therefore, the reaction tubes were arc-sealed. Powder X-ray diffraction was used to determine sample purity. The Rietveld refinement was conducted using the Jana2006 15 software on the powder pattern of the samples ( Figure S1).
Detailed information on synthesis, crystal structure determination, and physical properties determination can be found in the Supporting Information (see Experimental details). Details of the data collection and further crystallographic information are listed in Table 1 (IV) in the structure (Table 2).
Considering the metal positions only, the crystal structure of LiSr 2 [CrN 3 ] is related to the Li 3 Bi-type arrangement with the [CrN 3 ] 5− units forming a cubic closest packing pattern. All octahedral voids as well as half of the tetrahedral voids are occupied by Sr2, and Sr1, respectively, the remaining tetrahedral voids are filled with lithium. The whole setup is somewhat distorted (Figure 3 16.32 2θ range (deg) 8  The ionic part of the atomic interactions in the title compound was studied by computing the charge transfer. The application of the QTAIM method 23 yields the following effective charges: both Sr atoms are 1.4+, Cr is 1.0+, Li is 0.8+, N1 is 1.5−, and N2 and N3 are 1.6−. Additionally, the shapes of the QTAIM basins provide qualitative information on the nature of bonding for each atom ( Figure 5A). If the electrons of the last shell are completely transferred to the other atoms, the QTAIM shape involves the inner shells only, and the QTAIM basin assumes a round, spherical shape. The QTAIM basin of an atom participating in mainly two-atomic covalent bonds presents planar or almost planar faces that are perpendicular to the interatomic line between the bonded atoms. The N and Cr QTAIM basins shown in Figure 5A are good examples for the latter case. The Cr QTAIM basin was plotted in transparent mode so that Cr−N interatomic lines are visible. It is noteworthy to observe that the concave surfaces of the Cr QTAIM basin facing the two Sr atoms suggest polar covalent Cr−Sr interactions. In line with the above explanation, the atom with the concave surface provides more electrons to the bond than the neighboring atom having the convex round surface.
A deeper understanding of the nature of atomic interactions can be achieved by the topological analysis of the electronlocalizability indicator in the ELI-D representation. The maxima (attractors) of the ELI-D in the valence region and the associated basins are used to identify the bonding interactions. The atomic interactions in LiSr 2 [CrN 3 ] can be divided into two groups ( Figure 5B,C). The first group belongs to the Cr−N     Figure 5B for N3, N2, and N1, respectively). Such merging behavior of bond attractors toward "lone-pair" attractors was already found not only in nitridometalates Li 6 Ca 2 [Mn 2 N 6 ], 24 Li 6 Sr 2 [Mn 2 N 6 ], 25 or ammonia molecule NH 3 , 26 but also in other substances with high electronegativity difference like Mg 3 Pt 2 , 27 Eu 3 Ga 2 , 28 and K 3 Bi 2 . 29 The basin populations are 6.83, 6.89, and 6.90 electrons for N1−Cr, N2−Cr, and N3−Cr basins, respectively. The bond polarities, estimated according to the position space approach, 30 are 0.936, 0.927, and 0.928 on the scale between 0 (covalent) and 1 (ionic) for N1−Cr, N2−Cr, and N3−Cr, respectively. N atoms provide about 92.7% and Cr 4.5% of these basin populations, with the remaining electrons being contributed by 3, 4, or 6 atoms, respectively. Note that the total number of electrons each N type has in the valence region is 6.36, 6.43, and 6.46 for N1, N2, and N3, respectively. The second group consists of two Cr-dominated interactions. One of these contains 0.81 electrons of which 0.59 (72.6%), 0.05 (5.4%), and 0.04 (4.2%) are provided by Cr, N3, and Sr1, respectively. The remaining contributions by 8 atoms (3 × N, 3 × Sr, and 2 × Li) add up to 0.13 electrons. We label this basin as Cr-dominated 11-atomic interaction. The other one is a 3atomic Cr−Sr2−Sr1 interaction (as hinted by the shape of the Cr atomic basin) with individual contributions of 0.24 (Cr), 0.02 (Sr2), and 0.02 (Sr1) electrons adding up to the total bond population of 0.28. Both of these interactions can also be regarded as lone-pair-like features due to the large Cr contributions. The attractors of these Cr-dominated interactions are located above and below the N1−N2−N3 plane, and the line connecting the attractors is perpendicular to this plane. Therefore, it is illuminating to analyze the electronic structure with the quantization axis (the z-axis for the orbital angular momentum operator) taken perpendicular to one of the two N1−N2−N3 planes in the unit cell. The remarkable result is that the band giving rise to the DOS between −0.43 and 0 eV (Fermi level set to 0 eV) is made up of mainly the Cr 4s and 3d (3z 2 −r 2 ) orbitals ( Figure S4). Identical results were reported earlier for Ba 3 [Cr IV N 3 ]H. 11 Using the fact that this band is well isolated from the rest, we computed the electron density due to only this band, ρ top . Integration of ρ top in the whole unit cell gives two electrons per formula unit. It can also be integrated separately inside each ELI-D basin to get its contribution to the electron populations of the bonds and core shells. Of these two electrons, 0.42 and 0.10 are found in the basins of the Cr-dominated 11atomic bond and Cr−Sr1−Sr2 bond, respectively, accounting for the 52 and 36% of the bond populations, respectively. Before we discuss the different roles of the 4s and 3d (3z 2 −r 2 ) orbitals, we recall that in the ELI-D analysis the 3d electrons of Cr appear in the core region. The reason is that ELI-D yields the shell structure of atoms and the d electrons of transition metals belong to the penultimate shell. Hence, the Cr contributions to the two Cr-dominated bonds must come mostly from the 4s electrons. Now, noticing that the 3d (3z 2 −r 2 ) orbital is essentially the only d orbital present in this energy range, a strong deviation of the ELI-D distribution in the penultimate shell (principal quantum number n = 3) from the spherical distribution is expected. Indeed, Figure 5C shows (i) two ELI-D localization domains along the local quantization axis on either side of the Cr atom, (ii) a three-lobed ELI-D localization domain around the Cr atom in the plane perpendicular to the quantization axis (dark-orange isosurfaces). The corresponding basins can be referred to as perpendicular outermost core basin and in-plane outermost core basin, respectively. Integrating ρ top inside these we find 0.50 and 0.30 electrons, respectively. These must be mainly due to the 3d (3z 2 −r 2 ) orbital.
In summary, the bonding in the anion [CrN 3 ] 5− is characterized by strongly polar Cr−N interaction, leading even to the suppressing of the dedicated ELI-D attraction and formation of the common "lone-pair" + bond basin and lonepair-like Cr-dominated multi-atom interactions.  Table S6).
The stretching modes of the anion [CrN 3 ] 5− are located at 822−827 cm −1 and at 821−841 cm −1 in Raman and IR spectra, respectively. Only four and two bands out of the expected six are distinguishable. In Raman spectrum, two bands at 400 and 345 cm −1 are observed, which were assigned to deformation modes. Additionally, the bands above 3500 cm −1 are noticeable in all measured samples and reflect overtone or combination bands ( Figure S5).
The phonon modes were calculated using the fully optimized unit cell. The optimized lattice parameters are a = 5.5712 Å, b = 5.4044 Å, and c = 7.5555 Å with β = 92.33°, yielding only 0.7% larger volume compared to the experimental value ( Table 1). The calculated phonon DOS is shown in Figure 6B. The modes due to the anion [CrN 3 ] 5− are the highest, lying between 810 and 900 cm −1 . These stretching modes are followed by the modes of the [LiN 3 ] units, located between 450 and 500 cm −1 and dominated by Li contributions (40−45% per Li atom). The modes involving the atoms of all three N types, Cr and Li, cover the range between 250 and 430 cm −1 . Being the heaviest atom, Sr contributes mainly to modes below 200 cm −1 .
The frequencies computed at the Γ point agree quite well with their experimental counterparts (Table S5).
Raman spectra were also measured for Sr 3 [Cr III N 3 ] to be used as a reference ( Figure 6C) (IV) in LiSr 2 [CrN 3 ] yields a stronger Cr−N interaction and therefore higher frequencies due to increased charge difference.
Magnetic Properties. The magnetization of the LiSr 2 [CrN 3 ] polycrystalline sample was measured for temperatures between T = 1.8 and 300 K in μ 0 H = 1, 3.5, and 7 T magnetic fields. A predominantly ferromagnetic signal with a Curie-type upturn at low temperatures is observed (Figure 7). The ferromagnetic behavior is indicated by the change of magnetic susceptibility χ(T) with field. The measured magnetization vs field M(H) at temperatures T = 2, 50, 150, and 300 K shows ferromagnetism with a narrow hysteresis and saturation of magnetization ( Figure S6). The difference in susceptibility at  Inorganic Chemistry pubs.acs.org/IC Article high fields corresponds to an equivalent amount of 0.026 at. % Fe, which could only be explained by presence of ferromagnetic impurities. After subtracting the ferromagnetic impurity and the sample holder contributions from the overall signal, the remaining signal is paramagnetic. By fitting the remaining signal with the Curie law, the effective magnetic moment value μ eff = 0.1 μ B is obtained. This value is too low to describe a paramagnetic Cr ion in the bulk of the sample and can only be attributed to a small amount of unidentified paramagnetic impurities close to the detection limit of PXRD. Therefore, we believe that LiSr 2 [CrN 3 ] is diamagnetic, which correlates well with both the expected absence of unpaired electrons in the nonmagnetic ground state of chromium (IV), and the results of the crystal structure refinement.